The effect of water activity on granule characteristics and tablet properties produced by moisture activated dry granulation (MADG)

• Effects of added water for MADG on the resulting granule characteristics and tablet properties were studied.
• MADG tablet tensile strength increased with increasing water activity from 0.0% to 2.5% added water.
• The excess amount of free water may have caused the decreasing tensile strength of the MADG tablets.
• MADG tablet initial wetting was correlated with water activity.
• Free water content was shown to have significant impact on the initial wetting of MADG tablets.

This study evaluated the effects of changing the amount of added water for moisture activated dry granulation (MADG) on the resulting granule characteristics and tablet properties. Water soluble fillers and binders were pre-mixed in a granulator, and the binder was activated by a small amount of water to form granules. The amount of added water was defined as the percentage of granulation water by mass to charged powder. Granules made with 0.0%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, and 5.0% (w/w) added water were evaluated in this study. Furthermore, the effect of free water content on tablet characteristics was investigated by measuring water activity.The Hausner ratio (HR) of MADG granules is significantly decreased by increasing the amount of added water up to 1.0% (water activity over 0.39). However, the HR of MADG granules did not change with further increase in water activity (added water over 2.5%). The free water content (water activity over 0.61, water added over 2.5%) may reach a limit in improving the flowability of MADG granules.MADG tablet tensile strength was increased along with the increased water activity when the added water amount was from 0.0% to 2.5%. However, MADG tablet tensile strength prepared with 3.0% or 5.0% added water was lower than in other tablets. Similarly, MADG tablet tensile strength was increased with increasing water activity from 0.0% to 2.5% added water. Yet, the magnitude of the negative slope between 2.5% and 5.0% added water was approximately ten times greater compared to the magnitude of the slope between 0.0% and 2.5% added water. By increasing the amount of added water over 2.5%, the excess amount of free water may have caused the decreasing tensile strength of the MADG tablets.MADG tablet initial wetting was correlated with water activity, and free water content was shown to have significant impact on the initial wetting of MADG tablets. Furthermore, this association between free water content and initial wetting of MADG tablets had a significant impact on tablet disintegration. Free water content had a low or minor impact on tablet porosity and capillary wetting.

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